1. Field of the Invention
This invention relates to novel quaternary phosphonium salts, and to methods for the preparation thereof. More particularly it relates to novel quaternary phosphonium salts in which each and every phosphorus atom bears a ureidomethyl substituent.
2. Description of the Prior Art
Polymers based on tetrakis(hydroxymethyl)phosphonium salts such as the chloride (THPC) and more recently the sulfate (THPS) are used extensively in flame retardant finishes for cotton (J. W. Lyons, "The Chemistry and Uses of Fire Retardants," Wiley-Interscience, New York, 1970, pp 189-208). The phosphonium salt, partially neutralized with a base such as triethanolamine or sodium hydroxide to prevent premature polymerization, is applied to the cotton with a multifunctional nitrogen compound such as urea or melamine and heat cured (THPC/amide process), or it is applied to the cotton by itself and ammonia cured (THPOH/ammonia process). In either case, the use of a base or buffer causes the phosphonium salt to dissociate to tris(hydroxymethyl)phosphine (THP) and formaldehyde, resulting in a finish that no longer has a quaternary phosphonium structure. THPC/urea, THPC/melamine, and THPC/ammonia polymers prepared in the solid phase or in solution lose some formaldehyde and all of the chlorine, indicating conversion of the THPC to THP [M.A. Kasem et al., J. Appl. Polym. Sci., 15, 2237 (1971)]. The presence of trivalent phosphorus in several THPC/amide and THPOH/ammonia finishes has been established by iodometric titration [D. J. Daigle et al., J. Fire Flam., 1, 178 (1970)].
In one variant of the THPOH/ammonia process, a precondensate of urea and THPC is prepared by condensing the reagents in a 0.5:1 molar ratio prior to application of the finish to the cotton (PROBAN process). Such precondensates fall outside the scope of the present invention because they contain residual hydroxymethyl groups. In fact, the retention of at least one free methylol (i.e, hydroxymethyl) group per phosphorus atom is considered essential for the precondensate to retain its polymer-forming ability [W. A. Reeves and J. D. Guthri, U.S. Pat. No. 2,772,188 (1956); H. Coates, U.S. Pat. No. 2,983,623 (1961)].
Condensation of THPC with 1,1-diphenylurea gives a quaternary ureidomethyl phosphonium salt having properties similar to some of those of the present invention, but alkyl ureas such as 1,1-dimethylurea or ethylene are said to yield only intractable oils [A. B. Pepperman, Jr. et al., J. Org. Chem., 41, 675 (1976)]. Moreover, TKP, a phosphate/acetate salt similar to THPC, is said not to react with 1,3-dimethylurea under the conditions used to prepare the precondensate with urea [A. Granzow, J. Amer. Chem. Soc., 99, 2648 (1977)].